In recent years, as higher performance and size reduction of electronic apparatuses have been demanded, producing electronic components with higher density and higher performance have been further demanded. By making electronic components smaller, high-functional, and more effective to address such demands, their temperature rises. This temperature rise poses a large problem. A method for increasing heat radiation of an electronic component has therefore become important. Description will be given below taking a light emitting diode (LED) as an example of the electronic component for which heat generation is problematic.
Among electronic components, an LED has characteristics that the amount of light emission decreases when the temperature rises too high. Therefore, heat radiation is essential for increasing the amount of light emission. As a technique for increasing heat radiation, a method is known that attaches an LED onto a metallic board and diffuses heat from the back surface of the LED.
FIG. 10 is a perspective view illustrating one example of a conventional heat radiating wiring board. In FIG. 10, lead frame 202 is embedded in resin 204. LED 206 and other components are to be mounted on lead frame 202. Here, heat radiation of LED 206 is transmitted through resin 204 to heat sink 208. In this way, heat is radiated through lead frame 202 and heat sink 208. Such a technique is described in Unexamined Japanese Patent Publication No. 2001-57408.
Here, if a large number of LEDs 206 are mounted and driven, a high current, e.g., of 30 to 150 A is required. To accept such a high current, the thickness (sectional area) of lead frame 202 needs to be increased. As a result, lead frame 202 needs to be thicker. However, if lead frame 202 is made thick, it becomes difficult to make lead frame 202 in a finer wiring shape by press working. Specifically, in press working of lead frame 202, the limit to making a finer shape is around the thickness of lead frame 202. That is, in the case of a thickness of 0.5 mm, the limit of the pattern width is 0.5 mm. It is very difficult with a thickness of 0.5 mm to make a fine pattern such as a pattern having a pattern width of 0.3 mm or 0.4 mm. To make the pattern width as in a typical print wiring circuit to 0.2 mm or 0.1 mm, the thickness of lead frame 202 needs to be reduced to 0.2 mm or 0.1 mm. With such a thickness (or sectional area), a high current for driving LED 206 cannot be accepted.
On the other hand, users have a need for LED 206 that has a high-level control circuit to realize light emission in accordance with applications. In such a case, a control circuit and a control semiconductor for LED 206 need to be mounted around LED 206. However, in a conventional high heat radiating board, since its circuit pattern constituted of lead frame 202 is a non-dense pattern provided for a high current, a dense pattern for mounting a semiconductor and the like cannot be formed. Therefore, a peripheral circuit of LED 206 cannot be surface mounted together with LED 206 on the same board, and is separately mounted on another board.
In particular, in the case of LEDs for various backlight and lighting, the needs for which have recently been increasing, it is necessary that LED 206 in array including a plurality of LEDs 206 are highly controlled by an electronic circuit. In aspects of low cost and compact size, it has been desired that a heat generating electronic component, such as LED 206, that requires a high current and heat radiation be mounted together with a general circuit component on the same board.
In the foregoing conventional structure, a high current, e.g., of 100 A (A stands for “ampere”, a unit used for measuring current) is required for driving LED 206. Further, for heat radiation of LED 206, a further increase in thickness of lead frame 202 has been promoted in a wiring board including lead frame 202. The resulting pattern of lead frame 202 is non-dense. Therefore, the conventional structure has a problem that it is difficult to mount a semiconductor circuit component for driving LED 206 together with LED 206 on the same board.